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Understanding quantum physics

  1. Jun 11, 2014 #1
    hi anyone or everyone on this forum
    can anyone answer this? from a quamtum perspective, is it correct to say that there is no up nor down, left or right, here or there, s'thing can be full&empty, can be dead&alive?

    Thank you very much indeed
  2. jcsd
  3. Jun 11, 2014 #2
    I doubt it.
    I'm thinking that when it comes to quantum mechanics, you aren't interested in any details. Would that be right?
    Or is it that you just don't know where to start?
  4. Jun 11, 2014 #3


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    Gold Member

    Been reading about Shroedinger's cat?

    QM is a system for calculations; in classical mechanics you can describe a system in terms of forces (Newton's original approach), or energy (as in Lagrangian and Hamiltonian formulations). The latter are usually studied in upper level physics courses because an understanding of basic mechanics in terms of forces is required.

    For example, you may want to solve for the motion of a system under particular conditions, while noting the stresses that occur instantaneously at all interconnections. This is useful in the design of mechanical systems, and today is much cheaper than building models and carrying out all of the corresponding test steps ... just run simulations until the design looks good; then build it and test it.

    Switching gears ...

    In QM you start by writing the Hamiltonian of the system of particles/atoms/experimental setup, just like you would in classical mechanics. Then the expressions for your variables are transformed from ordinary variables (e.g., position and momentum) into their corresponding quantum operators.

    This results in a differential equation for which the solution are the "eigenvectors and eigenvalues" of the "quantum wave" in its state space. With these solutions you can work out the expected results of the proposed experiment or device which is being designed. If it looks good ... then you build the device/apparatus and conduct experiments to make certain that everything works the way your model does; if not, we expect that the model was incorrect ... or your experiment was carried out incorrectly/with bad equipment/or wrong equipment/or a million other things that can go wrong!

    By refining the model, and improving the experimental technique, the model and experiment are brought into agreement.

    Note that if I build a model in classical mechanics, but ask questions about atoms ... the model and the experiment will seldom agree. If I tinker with the classical model a bit, and add some ideas from quantum physics ... then we have the state of quantum physics prior to Heisenberg/Shroedinger/Dirac ... and sometimes it works, sometimes not so good, and sometimes completely wrong.

    This process of problem/model/experiment results in continual improvement in our understanding of atoms, molecules, light, etc. With these results technology can be improved, and eventually new things can be manufactured and used by everyone - from computers (transistors, solid state manufacturing) to lasers (medical applications, laser pointers, metal fabrication ...), etc.

    Nowhere in this process are any of your questions relevant. They are essentially philosophical, and are motivated by a desire to "understand" QM in terms of our classical intuitions. This is why there are "interpretations" of QM, starting with Bohr's "correspondence principle", which appears in the calculations when the usual variables (position, momentum) are converted to operators. So it is an aid to doing the work, perhaps, but it doesn't get you very far in terms of:

    A. Building models that work
    B. Calculating the results of those models
    C. Experimental verification that the models actually work as expected

    This is the "shut up and compute" school of QM. It has very little to do with what is written in the pop sci books; it is the difference between doing and talking. :-)
  5. Jun 11, 2014 #4


    Staff: Mentor

    Just to add a bit to UltrafastPED's, SUPERB answer (good one :thumbs::thumbs::thumbs::thumbs:) you may like to see what the conceptual core of QM really is - for some reason most texts don't do it this way - don't really know why:

    If you want to understand Schroedinger's cat better do a post and myself and others will only be too happy to help. But just as an overview its solution in shut up and calculate is utterly trivial - a quantum observation occurred at the particle detector and from that point on everything is classical - the cat is alive or dead - period. There is an issue however, but its not what the pop-sci books will tell you - but do a post if you want to pursue it.

    Last edited: Jun 11, 2014
  6. Jun 12, 2014 #5
    giving thanks and some more questions

    Thanks a lot for your answers which have helped me a lot.
    Yes, I would like to better understand shrodinger's cat experiment, as well as the duality particle-wave. In which circumstances does light behave as wave or particle or both?o:)
  7. Jun 12, 2014 #6


    Staff: Mentor

    Do a post to understand Schrödinger's cat better. Happy to help with that one - will look out for your post - but just to whet your appetite its tied up with the modern view of what an observation is and decoherence.

    For wave particle duality check out the FAQ:

    Basically its a crock of the proverbial. It one of the halfway ideas on the way to the full blown theory that's no longer required once you have that machinery.

  8. Jun 12, 2014 #7
  9. Jun 14, 2014 #8
    It is certainly possible to understand what quantum physics is telling us about reality. I'll be exploring this for the layperson in my forthcoming book. For the academic version, see: http://www.cambridge.org/us/knowledge/discountpromotion/?site_locale=en_US&code=L2TIQM

    Basically, quantum states describe possibilities, while classical states describe actualities. The difference between the two is clearly definable in the transactional picture. For some introductory material, see my website.

  10. Jun 14, 2014 #9
    The purpose of the Schrodinger Cat thought experiment was basically to ask the question of how far can you really go with the notion that something doesn't happen until it is measured - or what does it take to make a measurement. If something can be in a superposition of several states, and each of those states has vastly different consequences, do you end up with a superposition of those different consequences? At some point somethings got to nail down one possibility out of many. Does it really take a person looking into the box to cause that to happen? But if it takes a person, does that mean that the universe operated differently before people existed?

    Most people would agree that the cat only has one fate - a fate which is determined before the box is opened. That would imply that everything that is needed to make a measurement was in that box with the cat. But exactly what is it that pulls particles out of the realm of quantum mechanics QM and forces them to be more specific? What exactly is the boundary between QM and the more familiar (if less accurate) rules that we are accustom to?

    Unfortunately, it's in the math. But at lest the cat makes it easier to ask the questions.

    As far you question on wave/particle duality, let's look at one photon at a time. Photomultiplier tubes can make individual photons visible. And when you see one, you are tempted to think that a photon traveled in a straight path from its source to the detection surface on the tube. But as you (or physicists) play with the photons path, and observe the results, a different picture emerges. It's as if the photon felt out a region of space and based on what it found in that space, it "picked" a place to land. That "feeling out" process is very similar to what a wave would do. But unlike a wave, it ends its journey at a single point.
  11. Jun 14, 2014 #10


    Staff: Mentor

    Actually that's not its issue from a theoretical viewpoint. It's when you have entangled states the wavefunction resides in an abstract infinite dimensional Hilbert space.

    At a less abstract level there are issues with the delayed choice experiment:

    'However, when experiments were finally devised that permitted both the double-slit version and the interferometer version of the experiment, it was conclusively shown that a photon could begin its life in an experimental configuration that would call for it to demonstrate its particle nature, end up in an experimental configuration that would call for it to demonstrate its wave nature, and that in these experiments it would always show its wave characteristics by interfering with itself. Furthermore, if the experiment was begun with the second beam-splitter in place but it was removed while the photon was in flight, then the photon would inevitably show up in a detector and not show any sign of interference effects. So the presence or absence of the second beam-splitter would always determine "wave or particle" manifestation. Many experimenters reached an interpretation of the experimental results that said that the change in final conditions would retroactively determine what the photon had "decided" to be as it was entering the first beam-splitter. As mentioned above, Wheeler rejected this interpretation.'

    The answer of course is its neither particle or wave - its quantum stuff and not easily analysable by such simplistic pictures.

  12. Jun 16, 2014 #11
    The question that was asked was when does a photon act like a particle and when like a wave. That's a simpler issue than entangled states.
    Why does everyone think it's appropriate to respond to a novice question like "is it correct to say that there is no up nor down" by jumping straight to Hilbert space? Are we deliberately making QM appear as intractable as possible?
  13. Jun 16, 2014 #12


    Staff: Mentor

    The answer is - its neither. It never is a particle and it never is a wave - its quantum stuff. This has been known since Dirac came up with his transformation theory in 1927. The FAQ's has the correct answer:
    So there is no duality – at least not within quantum mechanics. We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn’t mean that in physics, or in the working of physicists, such a duality has any significance.

    Because Scott it leads to wrong and woolly thinking, which is the precise reason the FAQ states it explicitly. I wish people in my early QM education had told me the truth so I didn't have to unlearn misconceptions later.

    Last edited: Jun 16, 2014
  14. Jun 20, 2014 #13
    Hi everyone,
    thanks a lot one more time, you're very kind indeed. As I said to Scott my question was "under what circunstances..."? This may or may not include time,temperature, etc bcause I don't know. Whether if I had asked When...? it would have only included time, I think. That is why I posed the question very consciously, and rejected using When. Do you see what I mean?
  15. Jun 20, 2014 #14
    Hi again
    Possibility and probability aren't the same thing, correct? For instance, is it possible for space debris to hit earth? Yes, probable? Not much. So, the cat in the box with the lid on is full of probabilities, while if opened the box by observer: just one possibility: the cat is alive and the observer interferes, and the cat is too big and warm for quantum reality, so the experiment cannot be done but as a hypothesis. Well, I hope u understand what I mean, bcause I have problems myself to put my thoughts into words
  16. Jun 20, 2014 #15


    Staff: Mentor

    Well actually they are.

    A rigorous development of probability is a difficult issue that will take us too far afield. If you are interested there is a relevant section in this forum.

    To start out simply view it as if you do a very large number of observations the proportion of each possible outcome will be its probability.

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